The need for precision positioning is well known in a number of mechanical and electronic arts. Close tolerance machining, for example, requires that the tool be positioned exactly. Various mechanisms for accomplishing such positioning, and various techniques for getting the most out of such mechanisms, are known.
A known mechanism for accomplishing precision positioning within a plane is the use of a linear motor to drive and hold a device in a particular position. Such linear motors, as well as electro-mechanical stepping motors, hydraulic actuators, and other mechanisms are known for use in precision positioning.
A typical solution to the need for high precision motion, in a plane of scanning for a robot end effector, is to provide a pair of linear actuators mechanically connected in tandem so that the Y-actuator is physically carried at the end of the X-actuator. A disadvantage of this approach is that the Y and X stages are mechanically in series, the Y stage moves the payload, but the X stage must move both the Y stage and the payload. Symmetry is broken, and in critical applications the control strategies for the X stage and the Y stage must be different for the different dynamics involved. Theta motion (rotation about an axis orthogonal to X and Y) may require still another tandem stage.
There are many applications where it is often necessary to move small payloads over limited distances with high speed and precision. Some of these applications include: scanning microscopy, lithography, robotic assembly and automatic testing.
In scanning microscopy, one must raster scan a sample beneath a small probe. The probe may be optical, capacitive, acoustic, magnetic, tunneling current, etc. The resulting time series signal becomes a two-dimensional image of the sample. In order to obtain the image in a very short time, the scanning stage must be exceedingly fast.
In lithography, one must quickly align a mask and substrate, involving precise motions in X, Y and theta.
Similar considerations apply to robotic assembly where two parts, or a part and a workpiece, must be aligned. In robotics, it is often desirable to tailor the compliance as well to accommodate misalignments and variation in parts dimensions. A promising technique in robotics is to use a combined coarse-fine motion approach where a fine positioning device is carried by a coarser positioning device, to achieve a large workspace envelope without sacrificing resolution.
In automatic testing, electrical or other kinds of probes are moved to discrete electrical connection pads on chips or modules whereupon a test is performed. In production, thousands of tests must be completed in a short time to make the process economically attractive.
This invention will fulfill requirements in applications where it is necessary to move light payloads over limited distances with high speed and precision.
Traditional solutions to this problem involve a stack of linear and rotary translation stages powered by motor-driven lead screws. The motors are either open-loop steppers or closed loop DC servos. Typically, either ball or crossed roller bearings are used for suspension. Recently, direct-drive linear motors have been used to eliminate the lead screws. These motion systems are all characterized by reasonably high accuracy, good load carrying capacity, but very low peak accelerations and velocities.
An improved motion device, light and small enough to be attached to the terminal link of an industrial robot was disclosed in U.S. Pat. No. 4,509,002, "Precision X-Y Positioner," by Ralph L. Hollis Jr., and U.S. Pat. No. 4,514,674, "Electromagnetic X-Y-Theta Precision Positioner," by Ralph L. Hollis Jr. and Bela L. Musits.
The device and several automation applications are described in R. H. Taylor, R. L. Hollis, and M. A. Lavin, "Precise Manipulation with Endpoint Sensing," presented at International Symposium on Robotics Research, Kyoto, Japan, Aug. 20-23, 1984, and published as an Engineering Technology Research Report No. RC 10670 (#47860) Aug. 7, 1984, which was later published in the IBM J. Res. Develop. Vol. 29, No. 4, pp. 363-376 (July, 1985).
Engineering details of the above-mentioned X-Y Positioner have been discussed by Ralph L. Hollis, in "A Planar XY Robotic Fine Positioning Device", Proc. 1985 IEEE International Conference on Robotics and Automation, St. Louis, Mo., pp. 329-336 (Mar. 25-28, 1985) and also in R. L. Hollis, "Design for a Planar XY Robotic Fine Positioning Device," PED-Vol. 15: Robotics and Manufacturing Automation, Winter Annual Mtg. of the ASME, Miami Beach, Fla., pp. 291-298 (Nov. 17-22, 1985).
R. L. Hollis, et al., "Robotic Circuit Board Testing Using Fine Positioners with Fiber-Optic Sensing," IBM Engineering Technology Research Report, No. RC 11164 (#50243) pp. 1-14 published on May 21, 1985, describes the use of a fine motion device to test electrical circuit boards.
The motion devices cited in these references exhibit moderately high speed and accelerations (240 mm/sec and 8.7 g force), high precision (0.5 microns motion resolution), limited travel (+/- 0.9 mm and +/- 2 degree.) and moderate payload capability (normally 1 Kg or less).
Additional work relating to the present invention has been disclosed by Kazuo Asakawa, Fumiaki Akiya, and Fumio Tabata, in their paper entitled "A variable Compliance Device and its Application for Automatic Assembly," Autofact 5 conference Proceedings, Detroit, Mich. pp. 10-1 to 10-17 (Nov. 14-17, 1983). These authors describe a compliance mechanism with 2 active and 3 passive degrees of freedom. The active degrees of freedom use planar coils. No positioning feedback is used, and their device is not used for positioning, although some force feedback is used.
Another, related reference is U.S. Pat. No. 4,874,998, issued on Oct. 17, 1989, entitled "Six Degree-of-Freedom Magnetically Levitated Fine Motion Device with Programmable Compliance", discloses a fine motion device having multiple degrees of freedom and being magnetically levitated. This fine motion device has programmable compliance as well as programmable motion.
Some of the common types of stages, such as, electro-magnetic stages, have been disclosed in some of the above-mentioned references. The stage of the present invention is a different type of a stage, it is an electro-dynamic stage. An electro-dynamic stage is a stage where electrical currents in moving coils directly interact with constant magnetic fields to produce motive forces. In contrast, an electro-magnetic stage is a stage where electric currents in fixed coils alter magnetic fields in permeable materials to produce motive forces. The electro-dynamic stage of this invention not only moves in the X and Y direction, but also rotates, and because the moving element is supported by air bearings there is no static friction.